The pulsation of arterial blood causes a pulsating volume variation of the arteries. If light emitted by a light source is transmitted through a part of the body being supplied with blood, the light is attenuated in dependence on the pulsation. The resulting variation in intensity of the light received by means of a sensor may, for example, be used for the determination of oxygen saturation of blood. The correlation between light absorption in the tissue and the composition and the thickness of the tissue through which the light passes is provided by Lambert-Beer's Law, and detection of scattering effects is provided by its modifications.
U.S. Pat. No. 4,109,643 discloses an apparatus for measuring the perfusion employing a light emitting diode for a measurement in the region of the fingertip. For this purpose, the light emitting diode is mounted on one side of a finger cap, and photo sensitive sensor is mounted on the opposite side. In this known apparatus, a change of the perfusion trend can be detected by means of electronic evaluation means, but no details may be obtained about the percentage volume change of the arterial blood upon a pulsation. As normal perfusion is not available before operations --e.g. caused by a shock condition of the patient, no exact statement may be provided about the condition of the patient during the operation by way of measuring the change in perfusion. A measurement of the change in perfusion is only useful if a correct reference value for the patient could be established before disturbance of perfusion occurred. In addition, measurement of perfusion may physiologically be used as an indication of the stress condition of a patient.